The generation of very short, for example, several picoseconds or less, pulses of coherent optical radiation is currently an area of technological interest. Such pulses would be useful in diverse practical applications such as high-speed optoelectronic devices and optical communication systems using extremely high data rates.
Several approaches have been taken in the attempt to produce very short optical pulses. One approach is mode locking in which a laser produces periodic pulses with a repetition rate related to the round-trip pulse transit time within the cavity. Both active and passive mode locking of diode lasers has been achieved. The former technique uses elements such as acousto-optic modulators to produce mode locking. The elements are, however, generally too slow to permit the attainment of the limiting pulse width.
Passive mode locking has achieved short pulse widths and it is believed that a saturable absorption region is required to obtain the shortest possible pulses. For example, a modified strip buried heterostructure AlGaAs diode laser with an external lens and mirror produced pulses having a duration of approximately 5.1 psec. See, for example, Applied Physics Letters, 37, pp. 267-269, Aug. 1, 1980. The shortest pulses reported were observed with diode lasers that had been aged until there was a noticeable increase in threshold current due to the development of dark line defects. It was hypothesized that the short pulse length was due, at least in part, to a saturable absorption loss produced as the aging process introduced dark line defects that caused saturable absorption. With strip buried heterostructure lasers that contain defects that were hypothesized to act as saturable absorbers, pulses as short as 1.3 psec were reported at the Eleventh International Quantum Electronics Conference, paper W.3., Boston, Mass., June 23-26, 1980.
Both of these reported lasers suffer drawbacks. For example, the concentration of what were believed to be saturable absorbers in the lasers increased with age and was therefore somewhat uncontrolled and variable. This, of course, led to devices having characteristics that changed significantly with time. Further, the presence of dark line defects resulted in accelerated degradation and failure of the lasers. Furthermore, since devices capable of generating still shorter pulses and having stable device characteristics are desirable, devices having a carefully controlled, i.e., stable with respect to time, saturable absorber region suitable for generating subpicosecond optical pulses and being introduced into the laser in a controlled and nondestructive manner are desirable.